Head-mounted displays (HMDs) have been in active development in the industry for well over twenty years to provide virtual reality immersion. Recently, these HMDs have been fitted with so called “combiners” to allow light from the outside world to mix with the computer generated images of virtual objects so as to give the impression that these virtual objects are “out there” visible in the world, and interacting with physical objects. As well as near-eye combiners, there are also projected AR systems in which images are projected from a head mounted system (HMPD) to be returned to a user's eyes by means of retroreflective screens.
FIG. 1 shows the arrangement of a typical prior art embodiment of a See Through Head Mounted Display (STHMD) from U.S. Pat. No. 6,049,429 that uses a freeform prism to direct computer generated images to the eye, and a “freeform corrector” as a combiner to allow images of the real world to be mixed.
FIG. 2 shows a prior art arrangement of an HMPD from U.S. Pat. No. 5,621,572, in which the computer images are projected from the head-mounted frame and reflected back into the eyes by means of a retroreflective screen. Here the images of real objects and virtual objects mix in front of the user.
FIG. 3 shows a flow chart of the pose calculation and render loop that is common to prior art augmented reality and mixed reality systems.
One of the most difficult technical requirements of these systems is to “register” virtual objects to fixed real objects or locations such that the virtual objects fit properly and appear to be stationary as the user moves his or her point of view. Many sources of registration error are known (see Holloway, Richard Lee. “Registration errors in augmented reality systems.” Dissertation University of North Carolina at Chapel Hill, 1995) and techniques are known for reducing these errors in systems with narrow field of view (FoV).
However, displays with wide FoV present more difficult registration situations. Embodiments of the present invention were developed in view of these issues.